The Braided World by Kay Kenyon

Heeding this cryptic message from deep space, the crew of the starship Restoration journeys from Earth to a distant planet, hoping to find humanity’s lost genetic diversity. But with the human race on the verge of extinction from the twin horrors of plague and a mysterious scourge of dark matter, how can an alien world harbor any remedies for Earth’s declining populations?

Worse, the Restoration arrives depleted: its captain is dead, its crew demoralized--except for an indomitable old woman whose power and wealth give her the privilege of naming the new captain. Anton Prados, a young, untested officer, will now preside over humanity’s first contact with an alien race. An alien race that, improbably, looks exactly like humans. Only, the Dassa possess highly unusual breeding habits--and a reproductive process that seems to be the nullification of all that is human. And they think much the same about humanity…